Field of the Invention
This invention relates to measuring carbon dioxide (CO2) dissolved in a solution. More particularly, this invention is related to the measuring of carbon dioxide dissolved in a solution after injection of carbon dioxide into a subterranean structure such as a saline aquifer.
State of the Art
At the current atmospheric concentrations, carbon dioxide is the largest contributor to radiative forcing in climate change. Atmospheric levels of carbon dioxide may be reduced through carbon dioxide capture, and injecting the captured carbon dioxide in geologically suitable sites. Among these sites, saline aquifers hold the highest promise for potential sequestration with a global storage capacity ranging from 1000 to 10,000 Gt (giga tonnes). In comparison, the annual anthropogenic emission of carbon dioxide is currently about 30 Gt (see IPCC report on underground geological storage).
Saline aquifers are subsurface geological formations consisting of water permeable rock that are saturated with brine. Super-critical carbon dioxide (i.e., carbon dioxide above critical pressure and temperature) can be injected into a saline aquifer via an injection well that extends into the saline aquifer. The injected carbon dioxide is retained in the saline aquifer via a combination of structural, residual, hysteretic, dissolution, and mineral storage mechanisms. For example, the injected carbon dioxide may either dissolve in the brine, react with the dissolved minerals or the surrounding rock, and/or become trapped in the pore space of the aquifer. Trapping occurs when carbon dioxide is surrounded by imbibing brine. Ideally, the saline aquifer has one or more layers of minimally porous and nearly impermeable rock (typically referred to as “cap-rock”) that prevents water flow and the escape of the injected carbon dioxide. Typically, cement is used to plug the injection well after injection and monitoring phases are complete.
For verification, and any subsequent decision-making on possible mitigation operations, it is critical to be able to validate the integrity of the sequestration of the injected carbon dioxide within the saline aquifer, i.e., the injected fluid is within the zone of containment. Current technologies employ surface soil and air sampling in order to detect potential carbon dioxide leakage, whilst changes in the formation are monitored using seismic, electromagnetic, neutron, and fluid sampling based monitoring within the rock formations. Except for direct sampling based inferences, monitoring methods are affected by the presence of a carbon dioxide rich phase and are largely insensitive to carbon dioxide dissolved in a liquid phase. A carbon dioxide rich phase is typically not present over large parts of the saline aquifer at late times. Furthermore, the arrival of carbon dioxide in supercritical state is preceded by brine with dissolved carbon dioxide. It is therefore useful to have a method that allows one to directly quantify the amount of carbon dioxide within a liquid phase. Currently, a dissolved carbon dioxide sensor deployable at downhole conditions is unavailable.